1. MATH 2311
Section 2.1

2. Counting Techniques
Combinatorics is the study of the number of ways a set of objects can be arranged, combined, or chosen; or the number of ways a succession of events can occur. Each result is called an outcome. An event is a subset of outcomes. When several events occur together, we have a compound event.
Outcome: all ways that something can happen.
Event: specific conditions about how something can happen.

3. The Fundamental Counting Principle
The Fundamental Counting Principle states that the total number of a ways a compound event may occur is n1  n2  n3 .... ni where n1 represents the number of ways the first event may occur, n2 represents the number of ways the second event may occur, and so on.

4. Example:
You are not allowed repeats (such as “double mushrooms”)
Example: How many ways can you create a pizza choosing a meat and two veggies if you have 3 choices of meats and 4 choices for veggies?

5. Rstudio and TI Commands:
Rstudio: factorials: (such as 5!) factorial(5) combinations: (such as 6C2) choose(6,2)
TI 83/84:
Select MATH  PRB (right arrow)
factorial: (option 4) 5!
permutation: (option 2)
6P2 will be written as 6 nPr 2
combination: (option 3)
6C2 will be written as 6 nCr 2

6. Permutations
A permutation of a set of n objects is an ordered arrangement of the objects. n Pn  n(n  1)(n  2)....3 2 1  n!
All objects are placed in order
Some of the objects are placed in order

7. Examples:
How many ways can six people be seated in a row? In how many ways can 3 of the six symbols, be arranged?
All six are being seated (factorial)
Since we are only arranging 3 out of the 6 options, this is a permutation.

8. With Repetition
When we allow repeated values, The number of orderings of n objects taken r at a time, with repetition is nr . Example: In how many ways can you write 4 letters on a tag using each of the letters C O U G A R with repetition?
n: number of items we are selecting from r: number of selections being made

9. Duplicate Objects
The number of permutations, P, of n objects taken n at a time with r objects alike, s of another kind alike, and t of another kind alike is
r, s, t, etc are the individual categories
n is the total number of items to select from
(r+s+t+….=n)

10. Example:
How many different words (they do not have to be real words) can be formed from the letters in the word MISSISSIPPI?

11. Circular Permutations
The number of circular permutations of n objects is (n – 1)!
Example:
In how many ways can 12 people be seated around a circular table?

12. Combinations:
Order does not matter
Treatment of each selection is the same.
A combination gives the number of ways of picking r unordered outcomes from n possibilities. The number of combinations of a set of n objects taken r at a time is

13. Example:
Creating a smaller group from a larger group: combination
How many ways can a committee of 5 be chosen from a group of 12 people?

14. Permutation or Combination:
How many ways can 1st, 2nd, and 3rd place ribbons be awarded when there are 15 contestants? How many ways can you be dealt a poker hand of 7 cards from a standard deck of 52? How many ways can a class President, Vice President, Secretary, Treasurer, and Historian be selected from a class of 500?
Different treatment of selections: permutation
No special treatment for different selections: combination
Each selection has a different treatment: permutation